Used with a 4JH4AE. 2009, 400 hours. Forward there seems to be no problems, but reverse is very weak, almost no power. It takes a really long time to stop the boat. I have tried with a 3-blade Flexofold and a VolvoPenta 3-blade. Both folding props. I have tried several boats with similar props, and they have much more power in reverse. So it must be the transmission. Any guess on the problem?

The Km35p is a cone clutchtransmission, and of course it could be that the cone is worn on the reverse side.

But a more likely explanation is that the transmission is available in two forward ratios, either 2.36:1 or 2.61:1, but the reverse ratio is always 3.16:1.

So regardless of which forward ratio you have, the propeller will always be turning slower in reverse for the same engineRPM. Couple this slower prop revolution with the lower efficiency when using the folding props in reverse and there is going to be less reverse thrust. The best solution is to give it more throttle.......

Thank you for your answer. More throttle is what i have done up to now, but it is not enough. My guess of the ratio forward/backwards is 1/10 or worse. If i go 7 knots and then go in reverse and up to 2500 rpm, it takes almost 60 seconds to stop the boat. Something must be wrong

What kind of boat do you have?
You might want to try a 3 blade fixed prop. If that does the trick, then it's not the trans and you'll just have to decide if you want the fixed prop or the longer stopping distance.

Do you have the YanmarSD50Saildrive in your boat? If so, you are probably experiencing a cone failure. Normally you lose forward with a right-hand prop, but if you have a left-hand prop, you will probably lose reverse.

Where is your boat located? I've been talking to a guy in the UAE that has a couple of spare SD50's that he took out of his catamaran.

My boat is, as i am, Danish. Built in 1990 after an Americas cup winner, Bianca 420.
I could try the fixed prop, but isnt there an easier way to check the transmission? I dont have a saildrive, but the KM35p gearbox.
Thanks

You can try the following to see if you have a slipping problem with the reverse gear of the transmission:

Tie the boat securely to the dock, start the engine, put the transmission in reverse and rev the engine up to around 2500 RPM. Use an optical tachometer to measure the RPM of the crankshaft and also the prop shaft RPM. Divide the crankshaft RPM by the reverse gear reduction ratio and that should equal the prop shaft RPM, or be very close to it. If the prop shaft RPM is much lower than the calculated figure, the transmission is slipping.

Thank you for your answer. More throttle is what i have done up to now, but it is not enough. My guess of the ratio forward/backwards is 1/10 or worse. If i go 7 knots and then go in reverse and up to 2500 rpm, it takes almost 60 seconds to stop the boat. Something must be wrong

What boat?

If I go 7 knots, I'm flying and it takes that much to stop my boat too.

The Km35p is a cone clutch transmission, and of course it could be that the cone is worn on the reverse side.

But a more likely explanation is that the transmission is available in two forward ratios, either 2.36:1 or 2.61:1, but the reverse ratio is always 3.16:1.

So regardless of which forward ratio you have, the propeller will always be turning slower in reverse for the same engine RPM. Couple this slower prop revolution with the lower efficiency when using the folding props in reverse and there is going to be less reverse thrust. The best solution is to give it more throttle.......

DougR

Huh?? WHo told you that

If say you buy a new prop and by mistake you have bought a left drive instead of a right drive, you can buy a new prop OR, much easier, just change the gear cables as forward and reverse are exactly the same revs and gear ratio.

I would not go for a fixed prop at all on this (lovely) boat, but you could go for a vaning prop. A compact one, like a Featherstream. You could even consider a 4 bladed version. They are truly excellent (I have fitted 3 of them now onto various sailing yachts).

If say you buy a new prop and by mistake you have bought a left drive instead of a right drive, you can buy a new prop OR, much easier, just change the gear cables as forward and reverse are exactly the same revs and gear ratio.

I would not go for a fixed prop at all on this (lovely) boat, but you could go for a vaning prop. A compact one, like a Featherstream. You could even consider a 4 bladed version. They are truly excellent (I have fitted 3 of them now onto various sailing yachts).

This problem can also be caused when the shift lever on the KM35 doesn't move fully into the reverse position. Check by removing the shift cable from the lever on the transmission. Manually move the transmission lever into reverse and make a note of the position of the lever. If the shift cable/linkage doesn't allow the lever to move fully into the reverse position, slippage can occur. Conversely, this can happen in forward too. Adjust the shift cable/linkage to allow full movement of the shift lever in both directions.

Sailmonkey; there is absolutely no problem in putting it in reverse at 7 knot. I am not smashing it from forward to reverse, but give it a little pause before reverse. The water will try to stop the blades from folding out, so no problem.
I have checked that the lever can move all the way. It can and does. Today i changed the oil to original Yanmar SAE 30. It felt a little better, but not sure, i have to try it tommorow.